The present invention relates to a new and improved method for the calibrational cross-sectional reduction of a workpiece which is in rotation during performance of the method.
Generally speaking, the method concerns the reduction of the cross-sectional area and the calibration of the outer diameter of a solid or hollow workpiece by cold forming or cold rolling, during which time the workpiece is axially advanced or fed along its workpiece axis or lengthwise axis and rotated about its workpiece axis, while the outer surface of the workpiece is machined by rolling tools at least at two locations which are situated opposite one another with respect to the workpiece axis. The rolling tools are rotatably mounted in two respective rolling heads each rotatably driven to rotate about a rolling head axis which is disposed transverse to the workpiece axis. The rolling heads move the rolling tools in a planetary-like revolving path of motion or travel. The rolling tools, by exerting a rapid sequence of blows or impacts, cold form or cold roll the outer surface of the workpiece such that the cold rolling operations which occur in succession at least predominately in the direction of the workpiece axis overlap in the workpiece-axial direction and in the workpiece circumferential direction and bring about a displacement of material which predominately occurs in the workpiece axial direction.
It is here mentioned that, for instance, for so-called torsion bars or rods which are used for very many different technical fields of application, such as for instance as resilient or spring rods, drive shafts and the like, apart from thicker portions also require thinner portions. Furthermore, for well known reasons there cannot be present any steps or step portions, rather there are required gradual transitions at the location of changes in diameter.
As a general rule, such torsion bars or rods are machined on a lathe such that they do not exhibit any imbalance or their properties otherwise do not exhibit any unfavorably influencing irregularities which could arise during their previous fabrication, for instance during forging. The machine lathing of such slim long parts is well known to be problematic, and the tool service life and working speeds are rather modest with the required hard materials. The cutting or removal of the material automatically leads to losses in material. There are required relatively large workpiece diameters. Also relatively great hardening distortions become problematic.
These drawbacks are avoided with the method disclosed in the commonly assigned Swiss Patent No. 658,006, granted Oct. 15, 1986 and which has been briefly set forth at the introduction of this disclosure. The reduction in the cross-sectional area is precisely accomplished by cold forming or cold rolling, that is to say by carrying out a cold impact rolling operation during which there cannot only be realized exceedingly high working speeds, but there can be complied with the purposes of the invention. The tool service life is very high without impairing the precision. High operating speeds allow for a material flow with at least in many cases permanent, i.e. through-going plastification, resulting in a much more extensive or deep reaching material strength. If a subsequent hardening operation is needed then there is only to be expected a modest hardening distortion. This constitutes a further advantage in contrast to machine turned or lathed parts. Since as a practical manner there are only machined rotating and uniformly driven parts the mass forces can be held within acceptable limits. Furthermore, there can be also easily avoided the oscillations or vibrations which are problematic during machine lathing.
If solid or hollow workpieces, for instance torsion rods or bars, were fabricated according to the method disclosed in the aforementioned Swiss Patent No. 658,006, there sometimes arose unexpected difficulties even when working with materials which were satisfactory for the intended purpose.
As to such difficulties, it is pointed out that in certain instances there could arise an insufficient plastification or plasticizing, whereas in other instances there was discerned a too pronounced increase in strength. In both cases the material flow was unsatisfactory and, in fact, there arose the danger of fracture.
Moreover, in the case of hollow, in other words tubular-like workpieces it sometimes was found that there existed an insufficient radial material flow. This possibly could lead to a poor forming or shaping of the internal profile. In order to improve the formation of the internal profile experience found it to be appropriate for the rolling heads to be more markedly advanced or infed. When there was finally attained a sufficient forming or shaping of the inner or internal profile, it was found that in certain instances the wall thickness was too thin.
In those instances no improvement could be attained by increasing the machining operation, in other words by increasing the density of the cold rolling operations.